gdb: add gdbarch_stack_grows_down function

In another patch I'm working on I needed to ask: does the stack grow
down, or grow up?

Looking around I found in infcall.c some code where we needed to ask
the same question, what we do there is ask:

  gdbarch_inner_than (gdbarch, 1, 2)

which should do the job.  However, I don't particularly like copying
this, it feels like we're asking something slightly different that
just happens to align with the question we're actually asking.

I propose adding a new function `gdbarch_stack_grows_down`.  This is
not going to be a gdbarch method that can be overridden, instead, this
will just call the gdbarch_inner_than function.  We already have some
gdbarch methods like this, checkout arch-utils.c for examples.

I think it's now clearer what we're actually doing.

A new self-test ensures that all architectures have a stack that
either grows down, or grows up.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>

3 files changed, 30 insertions, 6 deletions

diff --git a/gdb/gdbarch-selftests.c b/gdb/gdbarch-selftests.c
index 0dc0c50..db99fe0 100644
--- a/gdb/gdbarch-selftests.c
+++ b/gdb/gdbarch-selftests.c
@@ -164,6 +164,21 @@ register_name_test (struct gdbarch *gdbarch)
     }
 }
 
+/* Test gdbarch_stack_grows_down.  Stacks must either grow down or up.  */
+
+static void
+check_stack_growth (struct gdbarch *gdbarch)
+{
+  /* We don't call gdbarch_stack_grows_down here, instead we're testing the
+     implementation by calling gdbarch_inner_than.  GDB assumes that stacks
+     either grow down or up (see uses of gdbarch_stack_grows_down), so exactly
+     one of these needs to be true.  */
+  bool stack_grows_down = gdbarch_inner_than (gdbarch, 1, 2) != 0;
+  bool stack_grows_up = gdbarch_inner_than (gdbarch, 2, 1) != 0;
+
+  SELF_CHECK (stack_grows_up != stack_grows_down);
+}
+
 } // namespace selftests
 
 void _initialize_gdbarch_selftests ();
@@ -175,4 +190,7 @@ _initialize_gdbarch_selftests ()
 
   selftests::register_test_foreach_arch ("register_name",
 					 selftests::register_name_test);
+
+  selftests::register_test_foreach_arch ("stack_growth",
+					 selftests::check_stack_growth);
 }
diff --git a/gdb/gdbarch.h b/gdb/gdbarch.h
index 77d3406..d4c6795 100644
--- a/gdb/gdbarch.h
+++ b/gdb/gdbarch.h
@@ -370,4 +370,12 @@ gdbarch_num_cooked_regs (gdbarch *arch)
   return gdbarch_num_regs (arch) + gdbarch_num_pseudo_regs (arch);
 }
 
+/* Return true if stacks for ARCH grow down, otherwise return true.  */
+
+static inline bool
+gdbarch_stack_grows_down (gdbarch *arch)
+{
+  return gdbarch_inner_than (arch, 1, 2) != 0;
+}
+
 #endif
diff --git a/gdb/infcall.c b/gdb/infcall.c
index 23d5652..edac9a7 100644
--- a/gdb/infcall.c
+++ b/gdb/infcall.c
@@ -947,7 +947,7 @@ reserve_stack_space (const type *values_type, CORE_ADDR &sp)
   struct gdbarch *gdbarch = get_frame_arch (frame);
   CORE_ADDR addr = 0;
 
-  if (gdbarch_inner_than (gdbarch, 1, 2))
+  if (gdbarch_stack_grows_down (gdbarch))
     {
       /* Stack grows downward.  Align STRUCT_ADDR and SP after
 	 making space.  */
@@ -1128,7 +1128,7 @@ call_function_by_hand_dummy (struct value *function,
 	   address.  AMD64 called that region the "red zone".  Skip at
 	   least the "red zone" size before allocating any space on
 	   the stack.  */
-	if (gdbarch_inner_than (gdbarch, 1, 2))
+	if (gdbarch_stack_grows_down (gdbarch))
 	  sp -= gdbarch_frame_red_zone_size (gdbarch);
 	else
 	  sp += gdbarch_frame_red_zone_size (gdbarch);
@@ -1156,11 +1156,9 @@ call_function_by_hand_dummy (struct value *function,
 	   to pay :-).  */
 	if (sp == old_sp)
 	  {
-	    if (gdbarch_inner_than (gdbarch, 1, 2))
-	      /* Stack grows down.  */
+	    if (gdbarch_stack_grows_down (gdbarch))
 	      sp = gdbarch_frame_align (gdbarch, old_sp - 1);
 	    else
-	      /* Stack grows up.  */
 	      sp = gdbarch_frame_align (gdbarch, old_sp + 1);
 	  }
 	/* SP may have underflown address zero here from OLD_SP.  Memory access
@@ -1193,7 +1191,7 @@ call_function_by_hand_dummy (struct value *function,
 	  {
 	    CORE_ADDR lastval_addr = lastval->address ();
 
-	    if (gdbarch_inner_than (gdbarch, 1, 2))
+	    if (gdbarch_stack_grows_down (gdbarch))
 	      {
 		gdb_assert (sp >= lastval_addr);
 		sp = lastval_addr;